Mixing system for floating roof tanks

ABSTRACT

Floating roof, liquid storage tanks such as for storage of petroleum products have a mixer motor in the center of the roof with a shaft extending through the roof, and a vertically pumping impeller arranged below the roof in the liquid to mix the liquid in the tank. This replaces and has many advantages over side entry mixers for similar tanks.

United States Patent 1191 1111 3,762,690 Harrington 1 Oct. 2, 1973 [54]MIXING SYSTEM FOR FLOATING ROOF 3,411,756 11/1968 Ziegler 259/122 TANKS2,931,534 4/1960 Wiggins. 220/26 R 3,329,301 7/1967 Lee 220/26 R [75]Inventor: H. Richard Harrington, Doylestown,

Pa. Primary ExaminerR0bert W. Jenkins [73] Asslgneez :lliilllg EquipmentCo., Rochester, Attorney cumpston Shaw & Stephens [22] Filed: Jan.31,1972

[21] Appl. No.: 221,966 [57] ABSTRACT Floating roof, liquid storagetanks such as for storage [52] US. Cl. 259/108, 220/26 R ofpetroleumproducts have a mixer motor in the center [51] IIIL Cl Bolf 7/22 of theroof with a Shaft extending through the roof7 and [58] Field of Search259/121, 122, 124, a vertically pumping impeller arranged below the roof259/] 1081 107; 220/26 R in the liquid to mix the liquid in the tank.This replaces and has many advantages over side entry mixers for [56]References Cited Similar tanks UNlTED STATES PATENTS 3,343,817 9/1967Carangelo 259/122 10 Claims, 4 Drawing Figures PATENTED W 3.762.690

FIG- 2 FIG. 4

MIXING SYSTEM FOR FLOATING ROOF TANKS THE INVENTIVE IMPROVEMENT For manyyears it has been standard practice to use mixers in large tanks forstoring petroleum products. The mixers keep sediment from settling andhelp keep the tank clean and serviceable. It has also been standardpractice for many years that storage tank mixers were mounted on theside of the tank near the bottom and were flange-mounted through thetank wall. This created weaknesses and stress concentrations in thelower tank wall and threatened ruptures that would cause not only lossof the stored products, but also damage to the environment.Nevertheless, the prob lems of side entering mixers have been acceptedand dealt with successfully for many years.

For at least 30 years, large liquid storage tanks such as for petroleumproducts have also used floating roofs that float on top of the storedproduct and rise and fall with its level inside a tank rim. This hashitherto not been associated with the mixing problem within the tank orthe use of side-entry mixing units, except that such mixers interferewith maximum lowering of the roof, and increase the amount of unusabletank volume by a substantial amount.

The invention involves recognition of the possibility of mounting a tankmixer on the floating roof. At first impression, there were severalserious problems involved in such an arrangement, and it was initiallyvery doubtful that satisfactory mixing could be accomplished by aroof-mounted mixer. However, further study of the problem and laboratoryexperiments have shown that not only can satisfactory mixing beaccomplished with a roof-mounted mixer, but safety requirements can bemet, mounting and other problems can be solved, and the result has manysurprising economic advantages over side-entry mixers.

SUMMARY OF THE INVENTION The inventive mixing system applies to a liquidstorage tank having a floating roof, and it includes a mixer motormounted on top of the roof over a through opening in the roof. Agenerally vertical'shaft turned by the motor extends through the roofopening into the liquid below, and an impeller is mounted on the shaftbelow the roofin the liquid for mixing the liquid. The impeller isconfigured to pump predominantly vertically to establish a vertical flowpreferably in the center of the tank to induce radial flow along the topand bottom of the tank for full circulation. Flexible power connectionslead to the motor, and a vapor lock is preferably formed between theroof and the shaft.

DRAWINGS FIG. I is a partially schematic, sectional elevational view ofthe interior ofa liquid storage tank using the inventive mixing system;

FIG. 2 is a partially schematic, partially cut-away elevational view ofa vapor lock for the mixer shaft of FIG. 2; and

FIGS. 3 and 4 are partially schematic, fragmentary views of preferredliquid flow patterns for the inventive system.

DETAILED DESCRIPTION As shown in FIG. 1, storage tank is generally knownand preferred for storing petroleum products. It

has a floating roof 1] that is also generally known for floating on thestored product and rising and falling with its level. Heretofore, liquidmixing within tank 10 was accomplished by side-entering mixers mountedin the lower wall of tank 10, but by the inventive system, a mixer 12 ismounted on roof 11 as illustrated.

Mixer 12 includes a mixer motor 13 mounted on a plate 14 preferably inthe center of roof 11. Plate 14 covers a substantial central opening inroof 11, and as best shown in FIG. 2, has a small through opening 15 forreceiving the drive shaft 16 of mixer 12. A preferably liquid-filled,vapor lock 17 surrounds shaft 16 for a vapor seal between shaft 16 androof 11 at opening 15.

Shaft 16 extends below roof 11 into the liquid in tank 10 and supportsan impeller 18 that is configured for vertical pumping. With mixer 12 inthe center of roof 11 as preferred, vertical pumping by impeller 18 cankeep the product in tank 10 well mixed and circulated. Plate 14 ispreferably removable and large enough so that mixer 12 can be removed bylifting impeller 18 up through roof 11 through the opening normallycoveredby plate 14. This can be accomplished with a lifting jib 19 suchas shown in FIG. 1. Alternatively, plate 14 can be omitted, and mixer 12with its shaft 16 and impeller 18 can be permanently installed in roof11 to be removed or repaired only when tank 10 is drained and opened forinspection and repair.

One problem with side-entering mixers for tank 10 was that roof 1] couldnot be lowered below the level of the mixer units. This produced arelatively large heel or unusable volume in the bottom of tank 10, andthis was a significant economic detriment. In circumstances requiringthat the heel of tank Ill) be kept to a minimum, a sump or well 20 ispreferably formed in the bottom center of tank 10 in registry withimpeller 18, so that roof 11 can sink to its lowermost position withoutany interference from impeller 18. Another way to keep the heel of tank10 to a minimum is to mount impeller 18 as close under roof 11 asfeasible.

Motor 13 is powered through a power transmission conduit 21 leading infrom outside tank 10. Motor 13 can be either electrical with conduit 21carrying energized electrical conductors, or motor 13 can be hydraulicwith conduit 21 carrying pipes filled with pressurized liquid. Eitherway, motor 13 rises and falls with roof 11 relative to tank 10 torequire flexing of conduit 21 between tank 10 and motor 13. Severalforms of flexible conduit are available, and there are several ways thatcan be arranged to carry power safelyto motor 13. Many safetyrequirements have to be met, and especially for an electric motor 13,electric or spark hazards must be avoided.

One preferred solution to the flexible conduit problem is shown in FIG.1 where conduit 21 is fixed in place and rigid from motor 13 along thetop of roof 11, along ladder 22, and along the outside of tank 10.Ladder 22 moves between top and bottom positions of roof 1] asillustrated and provides part of the conduit connection.

At the top of ladder 22 a flexible conduit connection includes a fixedpost 23 having a swivel top 24 supporting a section of flexible conduit25 connecting to rigid conduit 21 at the upper end of ladder 22.Flexible conduit 25 and swivel top 24 allow the illustrated conduitmovement between the upper and lower positions of ladder 22 asillustrated.

At the bottom of ladder 22 on tank top 11, a rigid post 26 leads conduit21 to a swivel top 27 from which a length of flexible conduit 28 leadsto a support post 29 at the bottom of ladder 22. Flexible conduitsection 28 moves between upper and lower positions of ladder 22 asillustrated, to keep a free loop of conduit between posts 26 and 29 andsafely out of contact with other equipment to allow rise and fall ofroof Ill without hazard.

The circulation flow created in tank It) by impeller 18 is schematicallyrepresented in FIGS. 3 and 41 showing two alternative preferredembodiments. In FIG. 3, impeller 18 is oriented to pump upward to drawliquid up from the bottom of tank it) in a central column that isdirected against the under side of roofllll and forced radially outwardacross the top of tank 10. This causes a downward flow around the wallsof tank It) and an inwardly radial flow across the bottom of tank backto the uprising central flow to impeller 18. The result keeps the liquidin tank well mixed and prevents sediment settling. Such flow can beassisted by a draft tube and diffuser 30 arranged around impeller 18 asillustrated to help guide upflowing liquid into impeller 18 and to helpspeed the radially outward flow under roof 11. Draft tube and diffuser30 can be omitted, and this is preferred, especially for minimumprojection of mixing equipment down under roof 11.

The impeller 18 of FIG. 4 is oriented for downward pumping to establisha circulation pattern the reverse of FIG. 3. In FIG. 4, impeller 18draws liquid radially inward under roof 11 and pumps liquid downward tothe bottom center of tank 10 where the liquid turns radially outward tosweep across the bottom of tank 10 and rise along the outer walls to bedrawn radially inward under roof 11 for another pass through impeller18. The down-pumping impeller of FIG. 43 requires a little moreclearance under roof l1 and is preferably spaced'below roof II by aboutits radius.

In addition to the advantages already mentioned, the inventiveroof-mounted mixer system is superior to side-entry mixers in many ways.A well-designed mixer system in the center of a tank roof can mix theentire tank with one mixing unit, where the same size tank could requireseveral side-entry mixers. The installation of a roof mixer is muchsimpler and safer than a side entry mixer which requires a very strongand well constructed flange mounting with a secure stuffing box forminga liquid seal around the mixer shaft. The roofmounted unit is alsoeasier to maintain, partly because only a vapor lock is required aroundits shaft, and it can be removed without draining the tank. Side-entrymixers require a much higher minimum roof clearance above the bottom oftank 10 to make a much larger and more costly tank heel of unusablevolume.

Higher safety standards for petroleum storage tanks also makeroof-mounted mixer systems advantageous over side-entry mixers. In someplaces, a container wall is required around a petroleum storage tank, sothat the wall will contain all the petroleum that flows from a ruptureof a full tank. The container wall can sometimes be added mosteconomically as a double-walled tank. This greatly increases theproblems of mounting side-entry mixers, but does not affect roof-mountedmixer systems.

Once it has been established that a roof-mounted mixer system canaccomplish the desired mixing, the advantages in such an assembly areapparent and signif icant. However, this solution was overlooked formany years in spite of the advantages.

Persons wishing to practice the invention should remember that otherembodiments and variations can be adapted to particular circumstances.Even though one point of view is necessarily chosen in describing anddefining the invention, this should not inhibit broader or relatedembodiments going beyond the semantic orientation of this applicationbut falling within thespirit of the invention. For example, thoseskilled in the mixing art will appreciate the different mountings,conduit connections, impellers, draft tubes, and mixing devices usablein the inventive mixing system.

I claim:

1. An improved mixing system for a liquid storage tank having a floatingroof, said system comprising:

a. a mixer motor mounted on top of said floating roof to travel up anddown with said roof relative to said tank;

b. said roof having a through opening in the region of said motor;

c. a generally vertical shaft turned by said motor and extending throughsaid roof opening into said liquid;

d. an impeller on said shaft a predetermined distance below said roofinsaid liquid for mixing said liquid;

e. said impeller being configured to pump predominantly vertically insaid liquid;

f. means for supplying power to said motor from a remote source; and

g. said power supplying means including a power transmission line fixedto the outside of said tank, a power transmission line fixed relative tosaid floating roof, and a flexible power transmission line forconnecting said fixed lines to accommodate relative motion between saidfixed lines.

2. The system of claim I wherein said motor, said roof opening, and saidshaft are approximately in the center of said roof.

3. The system of claim 2 including a vapor lock between said roof andsaid shaft in the region of said opening.

4. The system of claim I wherein said motor is electric and said powertransmission lines comprise electric conduit.

5. The system of claim 1 wherein said motor is hydraulic and said powertransmission lines comprise hydraulic conduit.

6. The system of claim 1 including a vapor lock between said roof andsaid shaft in the region of said opening.

7. The system of claim 1 including a well formed in the bottom of saidtank below said impeller to receive' said impeller when said roof is inits lowermost position.

8. The system of claim 1 including a draft tube arranged under said roofaround said impeller.

9. The system of claim 8 wherein said impeller is oriented to pumpupward.

10. The system of claim 1 wherein said impeller is spaced below saidroof by approximately the radius of said impeller.

1. An improved mixing system for a liquid storage tank having a floatingroof, said system comprising: a. a mixer motor mounted on top of saidfloating roof to travel up and down with said roof relative to saidtank; b. said roof having a through opening in the region of said motor;c. a generally vertical shaft turned by said motor and extending throughsaid roof opening into said liquid; d. an impeller on said shAft apredetermined distance below said roof in said liquid for mixing saidliquid; e. said impeller being configured to pump predominantlyvertically in said liquid; f. means for supplying power to said motorfrom a remote source; and g. said power supplying means including apower transmission line fixed to the outside of said tank, a powertransmission line fixed relative to said floating roof, and a flexiblepower transmission line for connecting said fixed lines to accommodaterelative motion between said fixed lines.
 2. The system of claim 1wherein said motor, said roof opening, and said shaft are approximatelyin the center of said roof.
 3. The system of claim 2 including a vaporlock between said roof and said shaft in the region of said opening. 4.The system of claim 1 wherein said motor is electric and said powertransmission lines comprise electric conduit.
 5. The system of claim 1wherein said motor is hydraulic and said power transmission linescomprise hydraulic conduit.
 6. The system of claim 1 including a vaporlock between said roof and said shaft in the region of said opening. 7.The system of claim 1 including a well formed in the bottom of said tankbelow said impeller to receive said impeller when said roof is in itslowermost position.
 8. The system of claim 1 including a draft tubearranged under said roof around said impeller.
 9. The system of claim 8wherein said impeller is oriented to pump upward.
 10. The system ofclaim 1 wherein said impeller is spaced below said roof by approximatelythe radius of said impeller.